Electric



Aug. 20, 1957 G. H. JoNKER ETAL 2,803,803

ELECTRIC FILTER Filed Nov. 26, 1952 INVENTORS GERARD HEINRICH JONKERJOHANNES OTTO WOLTHUIS MAARTEN BANNINK AGEN T hfim Patented Aug. 20,1957 annotato FILTER Gerard Heinrich Jonker, Johannes Otto Wolthuis, andMaarten Bannink, Eindhoven, Netherlands, assignors, by mesneassignments, to North American Philips Company, Inc., New York, N. Y., acorporation of Deia= ware Application November 26, 1952, Serial No.322,614

Claims priority, application Netherlands January 17, 1952 2 Claims. (Cl.S33- 72) The invention relates to electric filters for electricoscillations corresponding with an audio frequency, particularly iiltersfor rejecting the interference tone (for example 9 kcs./s.) generated inthe output of a radio receiver by two transmitters adjacent inwavelength.

To reject such an interference tone use may be made or" a resonantcircuit tuned to the frequency to be rejected and made up of inductanceand capacitance. However, 1n general the use of such a resonant circuitis a comparatively expensive solution.

The object of this invention is to provide a generally cheaperalternative solution. The invention is characterized by the use of anelectrostrictive vibrator having a tubular or barshaped vibratory bodywhich is weighted the ends. The use of an electrostrictive vibrator forfiltering purposes is known per se for substantially higher frequencies.In accordance with this invention an electrostrictive vibrator is usedfor frequencies within the audio frequency range by loading a short,narrow tube consising of electrostrictive material with weights at itsen s.

The invention will be explained with reference to the accompanyingdrawing, in which:

Fig. l shows schematically one embodiment of this invention; and

Fig. 2 shows a longitudinal cross-sectional view of another embodimentof this invention.

Referring to Fig. l, the tubular body 4 is made of electrostrictivematerial. Associated with this vibrator body 4 is an internal electrode1 and two external electrodes Z and 3, providing terminals to which aninput potential may be applied between the internal electrode 1 and, forexample, the external electrode 2, and from which an output potentialmay be abstracted between the internal electrode 1 and the secondexternal electrode 3. The electrostrictive material is preferably anelectret, as is the case, for example, with barium metatitanate. After ahigh voltage has been applied for a short period of time between theelectrodes 1 and 2 and between the electrodes 1 and 3 it retains a givenremanent polarization. This polarization considerably increases theelectromechanical coupling.

If no further measures are taken and the input potential remainsconstant the potential produced across the output electrodes will be atmaximum value in the proximity of the mechanical resonance frequency ofthe vibratory body 4. Consequently the required rejection is thenobtainable by including the vibrator in a negative feedback circuit inthe radio receiver. However, at a rejection frequency of 9 kcS./s. thevibrator would thus be required to have a length of approximately 22 cm.

According t-o the invention a small construction of only a fewcentimeters is obtained because it proved possible to realize thevibratory body as a short narrow tube loaded with weights 5 at its ends.These weights, which together are usually considerably greater than thedead weight of the vibrator body 4, effect an imaginary elongation ofthe vibratory body 4 and consequently a reduction of its naturalresonance. This results in a considerable saving of space and ofelectrostrictive material.

It has been found that the device shown makes possible a circuit qualityQ of between and 300. Such high circuit quality is generally excessivefor the object aimed at by this invention. Therefore, according to afurther feature of the invention, means have been provided to adjust thedamping of the vibrator. To that end another set of weights (not shown)might, for example, be screwed against the weights 5 through dampingmaterial such as rubber discs with the thrust force being a measure ofthe damping effected by the rubber discs.

For this purpose, in the embodiment shown in Fig. 2 the referencenumerals of which designate similar construction elements as in Fig. l,rubber sockets 7 are arranged to surround the weights 5 so as to bringabout the required damping. This damping is adapted to be adjusted bygripping the sockets with an adjustable pressure by means of grooves 9recessed to the required depth in a metal screening can 8 and engaging arecess in the sockets 7.

Though, in principle, use may be made of a bar-shaped, for example, astrip-shaped, vibrator body in an exactly similar manner, the tubularvibrator body 4 has been found to be preferable owing to its greatermechanical strength.

While we have thus described our invention with specific examples andembodiments thereof, further modiiications will be readily apparent tothose skilled in the art without departing from the spirit and the scopeof the invention as defined in the appended claims.

What we claim is:

l. An electric filter for electrical oscillations within theaudio-frequency range, comprising a substantially cylindrical housinghaving a circumferential indentation formed substantially at each end ina side wall thereof, a pair of damping members mounted within thehousing at opposite ends thereof, a vibratory member mounted atlopposite ends within said damping members and having a resonancefrequency within said audio-frequency range, said vibratory membercomprising a tubular electrostrictive member having a given length andweight and a pair of weights mounted on and secured to opposite ends ofsaid electrostrictive member, the combined weight of said pair ofweights being greater than said given weight of said electrostrictivemember and the combined length of said pair of weights being less thansaid given length of said electrostrictive member, whereby the overalllength of said vibratory member is substantially less than the length ofan electrostrictive member alone resonant within said audio-frequencyrange, said damping members surrounding and engaging said weights andbeing engaged by said circumferential indentations to provide apredetermined pressure on said weights, and a plurality of spacedelectrodes coupled to said electrostrictive member to provide v.inputand output means therefor.

2. An electric filter as set forth in claim 1 wherein theelectrostrictive member is constituted of barium metatitanate, and thedamping members are constituted of rubber.

References Cited in the le of this patent UNITED STATES PATENTS 44Da..11y ",'`"2",'1." 4! Kuenstler Mar. 30, 1948 Diemer Nov. 22, 1949Hirsch June 2, 1953 Burns Aug. 4, 1953 Anthony et a1. Sept. 15, 1953FOREIGN PATENTS Germany Sept. 4, 1944 Great Britain Sept. 27, 1950 OTHERREFERENCES (Copy in S33-72.)

